The present invention relates to a process for preparing powders, and in particular to a process for preparing spherical hollow powders with superior homogeneity.
For better fabrication of electronic or structural ceramics, powder synthesis based on wet chemistry has gained wide attention these past few years. The high purity of starting materials, the molecular scale mixing of ingredients and the ability to control the composition precisely are all typical features of the solution process. As a consequence, better stoichiometric and microstructural control as well as extreme compositional homogeneity should be possible with the process. Various techniques using co-precipitation, sol-gel processing, freeze drying, controlled hydrolysis and high temperature pyrolysis, to name a few, were developed for the wet process. The aerosol technique, in which the well-mixed liquid precursor is nebulized into isoltated fine droplets and then reacted at a high temperature to form powders, has been evaluated to be beneficial in maintaining the homogeneity in the wet state and in eliminating the problems during calcination.
However, the homogeneity of powders prepared by the aerosol technique is still not satisfactory. For example, in the preparation of MgAl.sub.2 O.sub.4 powder by aerosol method, using nitrate solutions as precursors, partial separation of MgO on the surface of aerosolized powders was observed, and the chemical homogeneity could not be restored even subjected to a post heat treatment at temperature up to 1000.degree. C. Moreover, the shape of the particles produced by the aerosol technique are usually not perfectly spherical and the surfaces thereof are rough.